#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
#define pb push_back
#define ff first
#define ss second
int main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int n, m; cin>>n>>m;
int f = n + m;
vector<ll> p(f + 1);
vector<pii> g[f + 1];
for (int i = 2; i <= f; i++){
int x, w; cin>>x>>w;
g[x].pb({i, w});
p[i] = p[x] + w;
}
const int A = 1e3;
vector<vector<ll>> t(f + 1, vector<ll>(A + 1));
for (int i = n + 1; i <= f; i++){
for (int j = 0; j <= A; j++){
t[i][j] = abs(j - p[i]);
}
}
function<void(int)> fill = [&](int v){
if (v > n) return;
for (auto [i, w]: g[v]){
fill(i);
}
for (int x = 0; x <= A; x++){
for (auto [i, w]: g[v]){
ll mn = 1e18;
for (int a = 0; a <= min(A, w + x); a++){
mn = min(mn, t[i][a] + abs(a - x));
}
t[v][x] += mn;
}
}
};
fill(1);
ll out = 1e18;
for (int x = 0; x <= A; x++){
out = min(out, t[1][x]);
}
cout<<out<<"\n";
}
